Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 170651
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
851 PM HST Mon Oct 16 2017

.SYNOPSIS...
High pressure north of Hawaii will keep a windy trade wind
pattern in place through much of the work week, with the trades
trending down slightly Friday and weakening further over the
weekend. Clouds and showers will favor windward and mauka areas
through the period, but showers will reach leeward areas from
time to time as well due to the strength of the trades.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high centered 950 miles north
of Honolulu, is driving windy trades across the island chain this
evening. Infrared satellite imagery shows partly to mostly cloudy
skies in place across the State, with cloud cover most prevalent
over windward areas. Meanwhile, radar imagery shows scattered to
numerous showers moving into windward areas, with showers
occasionally reaching leeward areas as well. Main short term
concern over the next few days revolves around the strong trades.

High pressure will remain anchored to the north of the State through
mid-week, with another re-enforcing high building north of the
islands on Wednesday. This high will then remain nearly
stationary through Thursday, before shifting eastward and
weakening Friday and over the weekend as a cold front approaches
from the northwest. As a result, windy conditions are expected to
continue across the entire State through Thursday night, with the
trades easing a bit on Friday, and trending down further over the
weekend.

Winds have dropped below advisory levels for much of the State
this evening, so will cancel the Wind Advisory for most areas with
the evening update. For the typically windy areas such as Lanai,
Kahoolawe, as well as over and downwind of the Kohala mountains on
the Big Island, we will extend the Wind Advisory through Tuesday
evening. A Wind Advisory may also be needed for the central valley
of Maui Tuesday afternoon, but will re-evaluate this for the
morning forecast package. With the windy conditions persisting
through much of the work week, the Wind Advisory may need to be
extended and possibly expanded to cover other portions of the
State right through Thursday night.

As for sensible weather details, clouds and showers will favor
windward and mauka areas through the forecast period, with showers
also reaching leeward areas from time to time due to the strength of
the trades. Model solutions are indicating an increasing probability
that deep tropical moisture will get drawn northward into or near
the islands Sunday through early next week. As a result, will trend
toward a wetter forecast and continue to monitor model trends and
make adjustments as details become more clear.

&&

.AVIATION...
A strong high pressure center north of the Hawaiian
Islands will produce windy northeasterly trade winds through
Wednesday. Enough instability in the low levels will produce
scattered shower activity with brief periods of MVFR conditions
mainly over mountains and northeastern slopes of all islands.
Otherwise VFR conditions will prevail.

AIRMET Tango for low level mechanical turbulence remains in
effect over and south through west of all mountain areas due to
windy conditions.

&&

.MARINE...
Very strong and gusty trade winds will continue through mid-week
due to a strong surface high far north of the main Hawaiian
Islands. This high will be replaced by another strong high on
Wednesday, which will continue the gusty trades for an additional
day or two. The strong to gale-force trade winds are also
producing very rough and elevated seas around the islands. Gale
warnings are posted for the Pailolo and Alenuihaha channels
through Thursday afternoon, while a Small Craft Advisory remains
in effect for the remaining Hawaiian waters through the same time.
Trade winds speeds are expected to diminish slightly starting
Friday as the high starts to weaken and begins to move toward the
east.

Elevated, rough waves continue to produce dangerous conditions in
surf zones along most east facing shores. Therefore, a High Surf
Advisory remains in effect for these beaches through Wednesday
afternoon. Since the trade winds will likely continue to be
strong, this advisory will likely be extended from Wednesday
night through Thursday.

A long-period south swell is forecast to peak some time later
tonight or early Tuesday morning with a swell height of around 4
feet and a wave period near 17 seconds. A High Surf Advisory for
the south facing shores of all islands remains in effect through
Tuesday afternoon. This south swell will slowly diminish from
Tuesday night through Wednesday. Small southwest and south-
southwest swells arriving this weekend will produce small
background surf along south facing shores.

The current north-northwest swell is expected to gradually
subside through Wednesday. A long-period north-northwest swell
arriving Thursday is expected to peak Friday. This swell will
likely produce moderate-sized surf (below the High Surf Advisory
threshold) along most north facing shores, as well as along some
west facing shores of the smaller islands. A somewhat larger
northwest swell is due in Sunday night and Monday of next week,
with the surf potentially approaching High Surf Advisory levels
along the north and some west facing shores of the smaller
islands.

See the Oahu Surf Discussion (SRDHFO), which was updated earlier
this afternoon, for more detailed information regarding the swell
sources.

&&

.FIRE WEATHER...
Strong trade winds coupled with a drier airmass pushing over the
State will allow minimum relative humidity to drop into the 40 to
45 percent range during the afternoon hours over the next few
days. These conditions will result in an increased potential for
fires to ignite and spread quickly. The windy and relatively dry
conditions will continue through the end of the work week before
the trade winds weaken. The Keetch-Byram Drought Index, or KBDI,
is at 572 today (and would need to be at 600 for Red Flag Warning
criteria). Keep in mind that we will be close to these conditions
each afternoon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Tuesday for Niihau-Kauai
Leeward-Oahu South Shore-Waianae Coast-Molokai Leeward-Lanai
Makai-Kahoolawe-Maui Leeward West-Maui Central Valley-Leeward
Haleakala-Kona-Kohala.

High Surf Advisory until 6 PM HST Wednesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 10 PM HST Tuesday for Lanai-Kahoolawe-Big
Island North and East-Kohala.

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Gale Warning until 6 AM HST Thursday for Pailolo Channel-
Alenuihaha Channel.

&&

$$

DISCUSSION/FIRE WEATHER...Jelsema
AVIATION...Bohlin
MARINE...Houston


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